To promote the measures to prevent global warming by reducing CO2 emissions, a fuel-efficiency standard for the motorcar of 2015 has been formulated based on the Energy Saving Law. The improvement of the fuel efficiency has been thus urgently required. One of the methods for improving the fuel efficiency of the motorcar is to reduce the torque of bearings used for automotive wheels (which will be hereinafter referred to as hub unit bearings).
The hub unit bearings, which are designed for the driving wheel and the follower wheel, can rotatably support a hub ring for fixing the automotive wheel via a double-row rolling bearing. For the structural reasons, the inner ring is generally rotated in the hub unit bearing for driving wheels; and either the inner ring or the outer ring is rotated in that for follower wheels.
The hub unit bearings are roughly divided into four groups: the first generation structure where a bearing for automotive wheel such as a double row angular rolling bearing or the like is fitted into a space between a knuckle for constituting a suspension and a hub ring; the second generation structure where a flange for fixing on a car body or a flange for attaching to a wheel is directly formed on the periphery of the external member; the third generation structure where one of the internal raceways is directly formed on the circumference of the hub ring; and the fourth generation structure where the respective internal raceways are directly formed on the circumference of the hub ring and the circumference of the external joint member of a constant velocity universal joint coupling.
The hub unit bearing is disposed at a portion readily exposed to muddy water and the like. Therefore, a sealing unit such as a seal material is used to tightly seal the space between the external member and the internal member. Typically, nitrile rubber (hereinafter also referred to as NBR) is widely used for the seal material in consideration of the oil resistance, wear resistance, heat resistance, workability, inexpensiveness and the like.
To cope with the reduction of torque in the hub unit bearing, the bearing itself may have various factors to be modified or the composition of grease employed may be changed. For example, one of the representative methods for reducing the torque in terms of the employed grease is to reduce the stirring resistance of the grease by using a base oil with a low kinematic viscosity for the grease composition (Hiroaki Ishikawa “Technical Trends and Tribology of Hub Unit Bearing.” Tribologist, vol. 54(9), 2009, pp. 580-585). However, when the kinematic viscosity of the base oil is decreased, there is the problem that satisfactory bearing life cannot be obtained because of deterioration of the oil film under the conditions of high temperatures although the reduction of torque can be achieved.
To solve the above-mentioned problem, base oils with high viscosity indexes, such as hydrocarbon synthetic oils and ester type synthetic oils may be employed to prevent the decrease of viscosity under elevated temperatures.
For the hydrocarbon synthetic oils, however, it is difficult to fully satisfy the bearing life because the heat-resistant stability is slightly poor. The ester type synthetic oils cause the problem that the oils tend to swell the seal material to impair the sealing performance although the heat-resistant stability is excellent (JP 2010-132746 A).